Transketolase kinetics. The slow reconstitution of the holoenzyme is due to rate-limiting dimerization of the subunits

The mechanism of a one-substrate transketolase reaction. Part II

In a recent paper, we showed the difference between the first stage of the one-substrate and the two-substrate transketolase reactions - the possibility of transfer of glycolaldehyde formed as a result of cleavage of the donor substrate from the thiazole ring of thiamine diphosphate to its aminopyrimidine ring through the tricycle formation stage, which is necessary for binding and splitting the second molecule of donor substrate [O.N. Solovjeva et al., The mechanism of a one-substrate transketolase reaction, Biosci. Rep. 40 (8) (2020) BSR20180246]. Here we show that under the action of the reducing agent a tricycle accumulates in a significant amount. Therefore, a significant decrease in the reaction rate of the one-substrate transketolase reaction compared to the two-substrate reaction is due to the stage of transferring the first glycolaldehyde molecule from the thiazole ring to the aminopyrimidine ring of thiamine diphosphate. Fragmentation of the four-carbon thiamine diphosphate derivatives showed that two glycolaldehyde molecules are bound to both coenzyme rings and the erythrulose molecule is bound to a thiazole ring. It was concluded that in the one-substrate reaction erythrulose is formed on the thiazole ring of thiamine diphosphate from two glycol aldehyde molecules linked to both thiamine diphosphate rings. The kinetic characteristics were determined for the two substrates, fructose 6-phosphate and glycolaldehyde.

Fine-tuning the activity and stability of an evolved enzyme active-site through noncanonical amino-acids

Site-specific saturation mutagenesis within enzyme active sites can radically alter reaction specificity, though often with a trade-off in stability. Extending saturation mutagenesis with a range of noncanonical amino acids (ncAA) potentially increases the ability to improve activity and stability simultaneously. Previously, an Escherichia coli transketolase variant (S385Y/D469T/R520Q) was evolved to accept aromatic aldehydes not converted by wild-type. The aromatic residue Y385 was critical to the new acceptor substrate binding, and so was explored here beyond the natural aromatic residues, to probe side chain structure and electronics effects on enzyme function and stability. A series of five variants introduced decreasing aromatic ring electron density at position 385 in the order para-aminophenylalanine (pAMF), tyrosine (Y), phenylalanine (F), para-cyanophenylalanine (pCNF) and para-nitrophenylalanine (pNTF), and simultaneously modified the hydrogen-bonding potential of the aromatic substituent from accepting to donating. The fine-tuning of residue 385 yielded variants with a 43-fold increase in specific activity for 50 mm 3-HBA and 100% increased k_cat (pCNF), 290% improvement in K_m (pNTF), 240% improvement in k_cat /K_m (pAMF) and decreased substrate inhibition relative to Y. Structural modelling suggested switching of the ring-substituted functional group, from donating to accepting, stabilised a helix-turn (D259-H261) through an intersubunit H-bond with G262, to give a 7.8 °C increase in the thermal transition mid-point, T_m , and improved packing of pAMF. This is one of the first examples in which both catalytic activity and stability are simultaneously improved via site-specific ncAA incorporation into an enzyme active site, and further demonstrates the benefits of expanding designer libraries to include ncAAs.

The Pentose Phosphate Pathway Dynamics in Cancer and Its Dependency on Intracellular pH

The Pentose Phosphate Pathway (PPP) is one of the key metabolic pathways occurring in living cells to produce energy and maintain cellular homeostasis. Cancer cells have higher cytoplasmic utilization of glucose (glycolysis), even in the presence of oxygen; this is known as the "Warburg Effect". However, cytoplasmic glucose utilization can also occur in cancer through the PPP. This pathway contributes to cancer cells by operating in many different ways: (i) as a defense mechanism via the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) to prevent apoptosis, (ii) as a provision for the maintenance of energy by intermediate glycolysis, (iii) by increasing genomic material to the cellular pool of nucleic acid bases, (iv) by promoting survival through increasing glycolysis, and so increasing acid production, and (v) by inducing cellular proliferation by the synthesis of nucleic acid, fatty acid, and amino acid. Each step of the PPP can be upregulated in some types of cancer but not in others. An interesting aspect of this metabolic pathway is the shared regulation of the glycolytic and PPP pathways by intracellular pH (pHi). Indeed, as with glycolysis, the optimum activity of the enzymes driving the PPP occurs at an alkaline pHi, which is compatible with the cytoplasmic pH of cancer cells. Here, we outline each step of the PPP and discuss its possible correlation with cancer.

Novel insights into transketolase activation by cofactor binding identifies two native species subpopulations

Transketolase (TK) cofactor binding has been studied extensively over many years, yet certain mysteries remain, such as a lack of consensus on the cooperativity of thiamine pyrophosphate (TPP) binding into the two active sites, in the presence and absence of the divalent cation, Mg²⁺. Using a novel fluorescence-based assay, we determined directly the dissociation constants and cooperativity of TPP binding and provide the first comprehensive study over a broad range of cofactor concentrations. We confirmed the high-affinity dissociation constants and revealed a dependence of both the affinity and cooperativity of binding on [Mg²⁺], which explained the previous lack of consensus. A second, discrete and previously uncharacterised low-affinity TPP binding-site was also observed, and hence indicated the existence of two forms of TK with high- (TK_high) and low-affinity (TK_low). The relative proportions of each dimer were independent of the monomer-dimer transition, as probed by analytical ultracentrifugation at various [TK]. Mass spectrometry revealed that chemical oxidation of TK_low led to the formation of TK_high, which was 22-fold more active than TK_low. Finally, we propose a two-species model of transketolase activation that describes the interconversions between apo-/holo-TK_high and TK_low, and the potential to significantly improve biocatalytic activity by populating only the most active form.

Structure and functioning mechanism of transketolase

Studies of thiamine diphosphate-dependent enzymes appear to have commenced in 1937, with the isolation of the coenzyme of yeast pyruvate decarboxylase, which was demonstrated to be a diphosphoric ester of thiamine. For quite a long time, these studies were largely focused on enzymes decarboxylating α-keto acids, such as pyruvate decarboxylase and pyruvate dehydrogenase complexes. Transketolase, discovered independently by Racker and Horecker in 1953 (and named by Racker) [1], did not receive much attention until 1992, when crystal X-ray structure analysis of the enzyme from Saccharomyces cerevisiae was performed [2]. These data, together with the results of site-directed mutagenesis, made it possible to understand in detail the mechanism of thiamine diphosphate-dependent catalysis. Some progress was also made in studies of the functional properties of transketolase. The last review on transketolase, which was fairly complete, appeared in 1998 [3]. Therefore, the publication of this paper should not seem premature.

Structural stability of E. coli transketolase to temperature and pH denaturation

We have previously shown that the denaturation of TK with urea follows a non-aggregating though irreversible denaturation pathway in which the cofactor binding appears to become altered but without dissociating, then followed at higher urea by partial denaturation of the homodimer prior to any further unfolding or dissociation of the two monomers. Urea is not typically present during biocatalysis, whereas access to TK enzymes that retain activity at increased temperature and extreme pH would be useful for operation under conditions that increase substrate and product stability or solubility. To provide further insight into the underlying causes of its deactivation in process conditions, we have characterised the effects of temperature and pH on the structure, stability, aggregation and activity of Escherichia coli transketolase. The activity of TK was initially found to progressively improve after pre-incubation at increasing temperatures. Loss of activity at higher temperature and low pH resulted primarily from protein denaturation and subsequent irreversible aggregation. By contrast, high pH resulted in the formation of a native-like state that was only partially inactive. The apo-TK enzyme structure content also increased at pH 9 to converge on that of the holo-TK. While cofactor dissociation was previously proposed for high pH deactivation, the observed structural changes in apo-TK but not holo-TK indicate a more complex mechanism.

Functional nonequivalence of transketolase active centers

Transketolase (TK, EC 2.2.1.1), the key enzyme of the non-oxidative branch of pentose phosphate pathway of hydrocarbon transformation, plays an important role in a system of substrate rearrangement between pentose shunt and glycolysis, acting as a reversible link between the two metabolic pathways. In addition, it supplies precursors for biosyntheses of nucleotides, aromatic amino acids, and vitamins. In plants, the enzyme plays a central role in the Calvin cycle. TK catalyzes interconversion of sugar phosphates. Thiamine diphosphate (TDP) and bivalent cations serve as its cofactors. Being a typical TDP-dependent enzyme, TK is the least complex representative of this group of enzymes, and this accounts for its use as a model in studies of their structure and mechanism of action. TK is readily crystallized, this being the reason why the first crystal X-ray structure analysis of TDP-dependent enzymes was performed with a TK sample. Both the general structure of TK and the structures of its active centers have been studied in detail. In this article, we review experimental evidence of functional nonequivalence of the two active centers of TK, which are known to be identical by crystal X-ray structure analysis.

Structural stability of an enzyme biocatalyst

TK (transketolase) undergoes inactivation during biocatalytic processes due to oxidation, substrate and product inhibition, reactivity of aldehyde substrates, irreversible inactivation at low pH, and dissociation of cofactors. However, the contribution of protein denaturation to each of these mechanisms is not fully understood. The urea-induced reversible denaturations of the apo- and holo-enzyme forms of the homodimeric Escherichia coli TK have been characterized, along with the reconstitution of holo-TK from the apoenzyme and cofactors. An unusual cofactor-bound yet inactive intermediate occurs on both the reconstitution and holo-TK denaturation pathways. The denaturation pathways of the holo- and apoenzymes converge at a second intermediate consisting of a partially denatured apo-homodimer. Preliminary investigation of the denaturation under oxidizing conditions reveals further complexity in the mechanisms of enzyme deactivation that occur under biocatalytic conditions.

The two active sites in human branched-chain alpha-keto acid dehydrogenase operate independently without an obligatory alternating-site mechanism

A long standing controversy is whether an alternating activesite mechanism occurs during catalysis in thiamine diphosphate (ThDP)-dependent enzymes. We address this question by investigating the ThDP-dependent decarboxylase/dehydrogenase (E1b) component of the mitochondrial branched-chain alpha-keto acid dehydrogenase complex (BCKDC). Our crystal structure reveals that conformations of the two active sites in the human E1b heterotetramer harboring the reaction intermediate are identical. Acidic residues in the core of the E1b heterotetramer, which align with the proton-wire residues proposed to participate in active-site communication in the related pyruvate dehydrogenase from Bacillus stearothermophilus, are mutated. Enzyme kinetic data show that, except in a few cases because of protein misfolding, these alterations are largely without effect on overall activity of BCKDC, ruling out the requirement of a proton-relay mechanism in E1b. BCKDC overall activity is nullified at 50% phosphorylation of E1b, but it is restored to nearly half of the pre-phosphorylation level after dissociation and reconstitution of BCKDC with the same phosphorylated E1b. The results suggest that the abolition of overall activity likely results from the specific geometry of the half-phosphorylated E1b in the BCKDC assembly and not due to a disruption of the alternating active-site mechanism. Finally, we show that a mutant E1b containing only one functional active site exhibits half of the wild-type BCKDC activity, which directly argues against the obligatory communication between active sites. The above results provide evidence that the two active sites in the E1b heterotetramer operate independently during the ThDP-dependent decarboxylation reaction.

Inhibition of pyruvate decarboxylase from Z. mobilis by novel analogues of thiamine pyrophosphate: investigating pyrophosphate mimics

Replacement of the thiazolium ring of thiamine pyrophosphate with a triazole gives extremely potent inhibitors of pyruvate decarboxylase from Z. mobilis, with K(I) values down to 20 pM; this system was used to explore pyrophosphate mimics and several effective analogues were discovered.

Influence of donor substrate on kinetic parameters of thiamine diphosphate binding to transketolase

The two-step mechanism of interaction of thiamine diphosphate (ThDP) with transketolase (TK) has been studied: TK + ThDP <--> TK...ThDP <--> TK*-ThDP. The scheme involves the formation of inactive intermediate complex TK...ThDP followed by its transformation into catalytically active holoenzyme, TK*-ThDP. The dissociation and kinetic constants for individual stages of this process have been determined. The values of forward and backward rate constants change in the presence of the donor substrate hydroxypyruvate. This finally leads to an increase in the overall affinity of the coenzyme to TK.

Theoretical model of interactions between ligand-binding sites in a dimeric protein and its application for the analysis of thiamine diphosphate binding to yeast transketolase

The binding of thiamin diphosphate (ThDP) to yeast dimeric apotransketolase (apoTK) is accompanied by the appearance of a band in the absorption spectrum with maximum at 320 nm. The saturation function has been analyzed using a scheme that involves binding of ThDP to each subunit followed by the conformational transition of this subunit. It is assumed that the binding of ThDP to one subunit may affect the conformational transition of the other subunit. Rigorous mathematical expressions describing the dependence of the optical absorption on the total concentration of ThDP are first developed. Equilibrium constants and corresponding rate constants for the binding of ThDP to apoTK have been estimated. The negative cooperativity in the ThDP binding has been characterized by the function reflecting the dependence of the conformational change on the saturation of apoTK by ThDP.

Binding of the coenzyme and formation of the transketolase active center

Transketolase (TK) is a homodimer, the simplest representative of thiamine diphosphate (ThDP)-dependent enzymes. It was first ThDP-dependent enzymes the crystal structure of which has been solved and revealed the general fold for this class of enzymes and the interactions of the non-covalently bound coenzyme ThDP with the protein component. Transketolase is a convenient model to study the structure(s) of the active center and the mechanism of action of ThDP-dependent enzymes. This review summarizes the results of studies on the kinetics of the interaction of ThDP with TK from Saccharomyces cerevisiae as well as the generation of the catalytically active form of the coenzyme within the holoenzyme and formation of the enzyme's active center.

Size Matters for the Tripeptidylpeptidase II Complex from Drosophila

Tripeptidylpeptidase II (TPP II) is an exopeptidase of the subtilisin type of serine proteases, a key component of the protein degradation cascade in many eukaryotes, which cleaves tripeptides from the N terminus of proteasome-released products. The Drosophila TPP II is a large homooligomeric complex (approximately 6 MDa) that is organized in a unique repetitive structure with two strands each composed of ten stacked homodimers; two strands intertwine to form a spindle-shaped structure. We report a novel procedure of preparing an active, structurally homogeneous TPP II holo-complex overexpressed in Escherichia coli. Assembly studies revealed that the specific activity of TPP II increases with oligomer size, which in turn is strongly concentration-dependent. At a TPP II concentration such as prevailing in Drosophila, equilibration of size and activity proceeds on a time scale of hours and leads to spindle formation at a TPP II concentration of > or =0.03 mg/ml. Before equilibrium is reached, activation lags behind assembly, suggesting that activation occurs in a two-step process consisting of (i) assembly and (ii) a subsequent conformational change leading to a switch from basal to full activity. We propose a model consistent with the hyperbolic increase of activity with oligomer size. Spindle formation by strand pairing causes both significant thermodynamic and kinetic stabilization. The strands inherently heterogeneous in length are thus locked into a discrete oligomeric state. Our data indicate that the unique spindle form of the holo-complex represents an assembly motif stabilizing a highly active state.

Effects of transketolase cofactors on its conformation and stability

In studying transketolase (TK) from Saccharomyces cerevisiae, the majority of researchers use as cofactors Mg(2+) and thiamine diphosphate (ThDP) (by analogy with other ThDP-dependent enzymes), whereas the active site of native holoTK is known to contain only Ca(2+). Experiments in which Mg(2+) was substituted for Ca(2+) demonstrated that the kinetic properties of TK varied with the bivalent cation cofactor. This led to the assumption that TK species obtained by reconstitution from apoTK and ThDP in the presence of Ca(2+) or Mg(2+), respectively, adopt different conformations. Kinetic study of the H103A mutant yeast transketolase. FEBS Letters 567, 270-274]. Analysis of far-UV circular dichroism (CD) spectra and of data, obtained using methods of thermal denaturing, differential scanning calorimetry (DSC) and tryptophan fluorescence spectroscopy, corroborated this assumption. Indeed, the ratios of secondary structure elements in the molecule of apoTK, recorded in the presence of Ca(2+) or Mg(2+), respectively, turned out to be different. The two forms of the holoenzyme, obtained by reconstitution from apoTK and ThDP in the presence of Ca(2+) or Mg(2+), respectively, also differed in stability: the holoenzyme was more stable in the presence of Ca(2+) than Mg(2+).

Effect of Transketolase Substrates on Holoenzyme Reconstitution and Stability

The influence of transketolase substrates on the interaction of apotransketolase with its coenzyme thiamine diphosphate (TDP) and on the stability of the reconstituted holoenzyme was studied. Donor substrates increased the affinity of the coenzyme for transketolase, whereas acceptor substrate did not. In the presence of magnesium ions, the active centers of transketolase initially identical in TDP binding lose their equivalence in the presence of donor substrates. The stability of transketolase depended on the cation type used during its reconstitution--the holoenzyme reconstituted in the presence of calcium ions was more stable than the holoenzyme produced in the presence of magnesium ions. In the presence of donor substrate, the holoenzyme stability increased without depending on the cation used during the reconstitution. Donor substrate did not influence the interaction of apotransketolase with the inactive analog of the coenzyme N3'-pyridyl thiamine diphosphate and did not stabilize the transketolase complex with this analog. The findings suggest that the effect of the substrate on the interaction of the coenzyme with apotransketolase and on stability of the reconstituted holoenzyme is caused by generation of 2-(alpha,beta-dihydroxyethyl)thiamine diphosphate (an intermediate product of the transketolase reaction), which has higher affinity for apotransketolase than TDP.

Donor substrate regulation of transketolase

The influence of substrates on the interaction of apotransketolase with thiamin diphosphate was investigated in the presence of magnesium ions. It was shown that the donor substrates, but not the acceptor substrates, enhance the affinity of the coenzyme either to only one active center of transketolase or to both active centers, but to different degrees in each, resulting in a negative cooperativity for coenzyme binding. In the absence of donor substrate, negative cooperativity is not observed. The donor substrate did not affect the interaction of the apoenzyme with the inactive coenzyme analogue, N3'-pyridyl-thiamin diphosphate. The influence of the donor substrate on the coenzyme-apotransketolase interaction was predicted as a result of formation of the transketolase reaction intermediate 2-(alpha,beta-dihydroxyethyl)-thiamin diphosphate, which exhibited a higher affinity to the enzyme than thiamin diphosphate. The enhancement of thiamin diphosphate's affinity to apotransketolase in the presence of donor substrate is probably one of the mechanisms underlying the substrate-affected transketolase regulation at low coenzyme concentrations.

Kinetic study of the H103A mutant yeast transketolase

Data from site-directed mutagenesis and X-ray crystallography show that His103 of holotransketolase (holoTK) does not come into contact with thiamin diphosphate (ThDP) but stabilizes the transketolase (TK) reaction intermediate, alpha,beta-dihydroxyethyl-thiamin diphosphate, by forming a hydrogen bond with the oxygen of its beta-hydroxyethyl group [Eur. J. Biochem. 233 (1995) 750; Proc. Natl. Acad. Sci. USA 99 (2002) 591]. We studied the influence of His103 mutation on ThDP-binding and enzymatic activity. It was found that mutation does not affect the affinity of the coenzyme to apotransketolase (apoTK) in the presence of Ca(2+) (a cation found in the native holoenzyme) but changes all the kinetic parameters of the ThDP-apoTK interaction in the presence of Mg(2+) (a cation commonly used in ThDP-dependent enzymes studies). It was concluded that the structures of TK active centers formed in the presence of Mg(2+) and Ca(2+) are not identical. Mutation of His103 led to a significant acceleration of the one-substrate reaction but a slow down of the two-substrate reaction so that the rates of both types of catalysis became equal. Our results provide evidence for the intermediate-stabilizing function of His103.

Inhibition of transketolase by p-hydroxyphenylpyruvate

The effect of p-hydroxyphenylpyruvate, a natural analogue of transketolase substrate, on the catalytic activity of the enzyme was investigated. p-Hydroxyphenylpyruvate proved to be a reversible and competitive inhibitor of transketolase with respect to substrate; it was also able to displace thiamine diphosphate from holotransketolase. The data suggest that p-hydroxyphenylpyruvate participates in the regulation of tyrosine biosynthesis by influencing the catalytic activity of transketolase.

Characterization of point mutations in patients with pyruvate dehydrogenase deficiency: role of methionine-181, proline-188, and arginine-349 in the alpha subunit

Human pyruvate dehydrogenase (E1), a heterotetramer (alpha2beta2), is the first component of the pyruvate dehydrogenase complex (PDC). E1 catalyzes the thiamin pyrophosphate (TPP)-dependent decarboxylation of pyruvate and the reductive acetylation of the dihydrolipoamide acetyltransferase component. Site-directed mutagenesis was employed to recreate three point mutations in the alpha subunit identified in E1-deficient patients, M181V, R349H, and P188L (P188A mutant E1 was used because of the very low level of expression of P188L), to investigate the functional roles of these three amino acid residues. P188A mutant E1 was much less thermostable than the wild-type E1. The kcats of M181V and P188A mutant E1s determined in the PDC reaction were 38 and 24% of that of the wild-type enzyme, respectively. The apparent Km for TPP for M181V increased significantly (approx 250-fold when determined in the PDC assay), while the apparent Km for pyruvate increased by only about 3-fold. In contrast, P188A had similar Kms for the coenzyme and the substrate as the wild-type. Km values for R349H were not determined due to the extremely low activity of this mutant (1.2% of the wild-type E1-specific activity measured in the PDC assay). Wild-type E1 displayed a lag phase in the progress curve of the PDC reaction measured in the presence of low TPP concentrations (below 1 microM) only. All mutants had a lag phase that was not eliminated even at very high TPP concentrations, suggesting modifications in the conformation of the active site. Kinetic analysis indicated thiamin 2-thiothiazolone pyrophosphate (ThTTPP) to be an intermediate analog for wild-type human E1. M181V required a higher concentration of ThTTPP for inactivation than the wild-type and P188A E1s. The results of circular dichroism spectropolarimetry in the far UV region indicated that there were no major changes in the secondary structure of M181V, P188A, and R349H E1s. These mutant enzymes exhibited negative dichroic spectra at about 330 nm only in the presence of high TPP concentrations. This study suggests that arginine-349 is critical for E1's activity, methionine-181 is involved in the binding of TPP, and proline-188 is necessary for structural integrity of E1.

Properties and functions of the thiamin diphosphate dependent enzyme transketolase

This review highlights recent research on the properties and functions of the enzyme transketolase, which requires thiamin diphosphate and a divalent metal ion for its activity. The transketolase-catalysed reaction is part of the pentose phosphate pathway, where transketolase appears to control the non-oxidative branch of this pathway, although the overall flux of labelled substrates remains controversial. Yeast transketolase is one of several thiamin diphosphate dependent enzymes whose three-dimensional structures have been determined. Together with mutational analysis these structural data have led to detailed understanding of thiamin diphosphate catalysed reactions. In the homodimer transketolase the two catalytic sites, where dihydroxyethyl groups are transferred from ketose donors to aldose acceptors, are formed at the interface between the two subunits, where the thiazole and pyrimidine rings of thiamin diphosphate are bound. Transketolase is ubiquitous and more than 30 full-length sequences are known. The encoded protein sequences contain two motifs of high homology; one common to all thiamin diphosphate-dependent enzymes and the other a unique transketolase motif. All characterised transketolases have similar kinetic and physical properties, but the mammalian enzymes are more selective in substrate utilisation than the nonmammalian representatives. Since products of the transketolase-catalysed reaction serve as precursors for a number of synthetic compounds this enzyme has been exploited for industrial applications. Putative mutant forms of transketolase, once believed to predispose to disease, have not stood up to scrutiny. However, a modification of transketolase is a marker for Alzheimer's disease, and transketolase activity in erythrocytes is a measure of thiamin nutrition. The cornea contains a particularly high transketolase concentration, consistent with the proposal that pentose phosphate pathway activity has a role in the removal of light-generated radicals.

Cooperativity and flexibility of active sites in homodimeric transketolase

Here we summarize evidence for non-equivalence of two structurally similar active sites in transketolase and other thiamine-dependent enzymes. This non-equivalence takes place when the enzymes interact with various ligands (inhibitors, cations, coenzyme and substrates). Data on different strains in the structure of the holotransketolase subunits are also given. The above results are discussed within the framework of a concept of permanent alternative site oscillation of the transketolase molecule in the presence and in the absence of substrate as a manifestation of a 'flip-flop' mechanism.

Crystallography and mutagenesis of transketolase: mechanistic implications for enzymatic thiamin catalysis

The ThDP dependent enzyme transketolase is a convenient model system to study enzymatic thiamin catalysis. Crystallographic studies of the enzyme have identified the ThDP binding fold, the V-conformation of ThDP as the relevant conformation in enzymatic catalysis and details of enzyme-substrate interactions. Based on this structural information, the function of various active site residues in substrate binding and catalysis has been probed by site-directed mutagenesis.

Heterologous expression of human transketolase

Transketolase belongs to the family of thiamin diphosphate dependent enzymes. The aim of this study was to establish a bacterial expression system for human transketolase in order to investigate the functional characteristics of mammalian transketolases. The level of recombinant human enzyme expressed in Escherichia coli was modest. Purification of recombinant transketolase and separation from the E. coli enzyme has been greatly simplified by means of a non-cleavable hexa-histidine tag. The highest specific activity was 13.5 U/mg and the K(m) values were 0.27 +/- 0.02 and 0.51 +/- 0.05 mM for the substrates D-xylulose 5-phosphate and D-ribose 5-phosphate, respectively. Binding of cofactors to the apoenzyme showed the expected hysteresis. Without preincubation, the K(m) values for thiamin diphosphate and for Mg2+ were, respectively, 4.1 +/- 0.8 and 2.5 +/- 0.4 microM, but after 1 h of preincubation these values were 85 +/- 16 nM and 0.74 +/- 0.23 microM. The kinetic constants are similar to those of the native enzyme purified from human erythrocytes. Despite the modest expression level the reported system is well suited to a variety of functional studies.

Kinetic mechanism of active site non-equivalence in transketolase

The two-step mechanism of coenzyme (TDP) binding to apotransketolase has been examined by kinetic modeling, and the rate and equilibrium constants for each binding step for two active sites have been determined. The dissociation constants for the primary fast binding step and the forward rate constants for the secondary slow binding step have been shown to be similar for two active sites. The backward rate constants for the secondary binding step are different for two active sites, providing the kinetic mechanism of their non-equivalence in TDP binding.

Examination of the thiamin diphosphate binding site in yeast transketolase by site-directed mutagenesis

The role of two conserved amino acid residues in the thiamin diphosphate binding site of yeast transketolase has been analyzed by site-directed mutagenesis. Replacement of E162, which is part of a cluster of glutamic acid residues at the subunit interface, by alanine or glutamine results in mutant enzymes with most catalytic properties similar to wild-type enzyme. The two mutant enzymes show, however, significant increases in the K0.5 values for thiamin diphosphate in the absence of substrate and in the lag of the reaction progress curves. This suggests that the interaction of E162 with residue E418, and possibly E167, from the second subunit is important for formation and stabilization of the transketolase dimer. Replacement of the conserved residue D382, which is buried upon binding of thiamin diphosphate, by asparagine and alanine, results in mutant enzymes severely impaired in thiamin diphosphate binding and catalytic efficiency. The 25-80-fold increase in K0.5 for thiamin diphosphate suggests that D382 is involved in cofactor binding, probably by electrostatic compensation of the positive charge of the thiazolium ring and stabilization of a flexible loop at the active site. The decrease in catalytic activities in the D382 mutants indicates that this residue might also be important in subsequent steps in catalysis.

Effect of substitutions in the thiamin diphosphate-magnesium fold on the activation of the pyruvate dehydrogenase complex from Escherichia coli by cofactors and substrate

The homotropic regulation of the Escherichia coli pyruvate dehydrogenase multienzyme complex (PDHc) by its coenzyme thiamin diphosphate and its substrate pyruvate was re-examined with complexes containing three and one lipoyl domains per E2 chain, and several variants of the latter, containing substitutions in the putative thiamin diphosphate fold of E1 (G231A, G231S, C259S, C259N, and N258Q). It was found that all of the E1 variants had significantly reduced specific activities, as reported elsewhere (Russell, G. C., Machado, R. S., and Guest, J. R. (1992) Biochem. J. 287, 611-619). In addition, extensive kinetic studies were performed in an attempt to determine the effects of the amino acid substitutions on the Hill coefficients with respect to thiamin diphosphate and pyruvate. All but one of the variants were incapable of being saturated with thiamin diphosphate, even at concentrations > 5 mM. Most importantly, the striking activation lag phase lasting for many seconds in the parental complexes containing three and one lipoyl domains per E2 chain was totally eliminated in the variants. Furthermore, activation by the coenzyme was localized to the E1 subunit, because resolved E1 exhibits virtually the same behavior during the activation lag phase as does the complex. In the parental complexes two distinct lag phases could be resolved, the duration of both decreases with increasing ThDP concentration. A mechanism that is consistent with all of the kinetic data on the parental complexes involves rapid equilibration of the first ThDP with the E1 dimer, followed by a slow conformational equilibration, that in turn is followed by slow addition of the second ThDP to form the fully activated dimer. When the diphosphate site is badly impaired, the binding affinity is very much reduced, this perhaps eliminates the slow step leading to the activated dimer form of the E1.

Transketolase A of Escherichia coli K12. Purification and properties of the enzyme from recombinant strains

Transketolase A was purified to apparent homogeneity from recombinant Escherichia coli K12 cells carrying the homologous cloned tktA gene on a pUC19-derived plasmid. These recombinant cells exhibited a transketolase activity in crude extracts of up to 9.7 U/mg compared to < or = 0.1 U/mg in wild-type cells. Transketolase A was purified from crude extracts of a recombinant strain by successive ammonium sulfate precipitations and two anion-exchange chromatography steps (Q-Sepharose FF, Fractogel EMD-DEAE column) and afforded an apparently homogeneous protein band on SDS/PAGE. The enzyme, both in its active and apoform, had a molecular mass of 145,000 Da (+/- 10,000 Da), judged by gel-filtration chromatography. Subunits of 73,000 Da (+/- 2000 Da) were determined on SDS/PAGE, thus, transketolase A most likely forms a homodimer. N-terminal amino acid sequencing of the protein verified the identity with the cloned gene tktA. The specific activity of the purified enzyme, determined at 30 degrees C with the substrates xylulose 5-phosphate (donor of C2 compound) and ribose 5-phosphate (acceptor) at an optimal pH (50 mM glycylglycine, pH 8.5), was 50.4 U/mg. Km values for the substrates xylulose 5-phosphate and ribose 5-phosphate were 160 microM and 1.4 mM, respectively. Km values for the other physiological substrates of transketolase A were 90 microM for erythrose 4-phosphate (best acceptor substrate), 2.1 mM for D,L-glyceraldehyde 3-phosphate, 1.1 mM for fructose 6-phosphate, and 4 mM for sedoheptulose 7-phosphate. Hydroxypyruvate served as alternative donor (Km = 18 mM). Unphosphorylated acceptor compounds were formaldehyde (Km = 31 mM), glycolaldehyde (14 mM), D,L-glyceraldehyde (10 mM) and D-erythrose (150 mM). The enzyme was competitively inhibited by D-arabinose 5-phosphate (K = 6 mM at a concentration of 2.5 mM D-arabinose 5-phosphate) or by the chelating agent EDTA. The inactive apoform of transketolase A was yielded by dialysis against buffer containing 10 mM EDTA, thus removing the cofactors thiamine diphosphate and divalent cations. The reconstitution of the apoenzyme proceded faster in the presence of manganese ions (Kd = 7 microM at 10 microM thiamine diphosphate) than with other divalent cations.

Molecular genetics of transketolase in the pathogenesis of the Wernicke-Korsakoff syndrome

Thiamine deficiency, a frequent complication of alcoholism, plays an important role in the pathogenesis of the Wernicke-Korsakoff syndrome [WKS]. Previous work by a number of investigators has implicated the thiamine-utilizing enzyme transketolase [Tk] as being involved mechanistically in the genetic predisposition to WKS. In particular, Tk derived from fibroblasts has been found to have an increased Km app for its cofactor thiamine pyrophosphate [TPP] and/or exist in different isoelectric forms in alcoholic patients with WKS as compared with unaffected individuals. We have demonstrated that these differences are not due to different Tk alleles, tissue-specific Tk isozymes, or differential mRNA splicing. These findings point to other mechanisms to explain the biochemical Tk variants, such as differences in assembly of the functional holoenzyme or differences in modification of the primary translation product. Tk assembly or modification, once biochemically characterized, may be found to be subject to genetic variation.

Reconstitution of holotransketolase is by a thiamin-diphosphate-magnesium complex

When human erythrocyte apo-transketolase is mixed with cofactors and substrates, the progress curve exhibits a lag phase. Elimination of the lag phase requires the presence of saturating concentrations of cofactors, thiamin diphosphate and Mg2+. The most simple explanation of the observed hysteretic transition is that the slow binding of a Mg(2+)-thiamin-diphosphate species precedes slow isomerisation of the enzyme to the active form. Although the hysteretic transition involves more than one process, it does not involve the dissociation-association of enzyme subunits. The best estimate of the apparent Km, 1.59 +/- 0.23 microM, for the binding of Mg(2+)-thiamin diphosphate to transketolase was obtained in the presence of a high non-inhibitory concentration of magnesium and varied concentrations of thiamin diphosphate. Thus the reconstitution of the human enzyme differs from the yeast enzyme, which undergoes a rate-limiting dimerisation during reconstitution.

DNA sequence of the yeast transketolase gene

Transketolase (EC 2.2.1.1) is the enzyme that, together with aldolase, forms a reversible link between the glycolytic and pentose phosphate pathways. We have cloned and sequenced the transketolase gene from yeast (Saccharomyces cerevisiae). This is the first transketolase gene of the pentose phosphate shunt to be sequenced from any source. The molecular mass of the proposed translated protein is 73,976 daltons, in good agreement with the observed molecular mass of about 75,000 daltons. The 5'-nontranslated region of the gene is similar to other yeast genes. There is no evidence of 5'-splice junctions or branch points in the sequence. The 3'-nontranslated region contains the polyadenylation signal (AATAAA), 80 base pairs downstream from the termination codon. A high degree of homology is found between yeast transketolase and dihydroxyacetone synthase (formaldehyde transketolase) from the yeast Hansenula polymorpha. The overall sequence identity between these two proteins is 37%, with four regions of much greater similarity. The regions from amino acid residues 98-131, 157-182, 410-433, and 474-489 have sequence identities of 74%, 66%, 83%, and 82%, respectively. One of these regions (157-182) includes a possible thiamin pyrophosphate (TPP) binding domain, and another (410-433) may contain the catalytic domain.

Purification and partial characterization of pyruvate decarboxylase from Oryza sativa L

Pyruvate decarboxylase(PyrDC) was purified from rice bran to a specific activity of 1 mu kat/mg and partially characterized. The holoenzyme is a tetramer of two types of subunits with molecular masses 64 kDa and 62 kDa. Purified rice PyrDC exhibits positive cooperative kinetics with respect to pyruvate and functions with a significant lag phase. When compared to other plant PyrDC, the lag phase was shorter at low pyruvate concentrations and the S0.5 was smaller. The optimum pH (6.25) was also less acidic and the enzyme retained 30% of its maximal activity at neutral pH. In contrast to other plant PyrDC, rice PyrDC could be active at the onset of anoxia and would be activated by small changes in pyruvate concentration.

Studies on the nature of thiamine pyrophosphate binding and dependency on divalent cations of transketolase from human erythrocytes

1. The binding kinetics for [35S]thiamine pyrophosphate to transketolase and the dependency of transketolase on divalent cations for activity were investigated. 2. With Scatchard analysis, dissociation constant (Kd) and n value were calculated to be 0.2 x 10(-6) M and 0.66 respectively. 3. The activity of the reconstituted enzyme increased in the order of Co2+ less than Mn2+ less than Ca2+ less than Mg2+. The native transketolase contained Mg2+ in its molecular structure.

Measurement of Michaelis constant for human erythrocyte transketolase and thiamin diphosphate

Human erythrocyte transketolase could be resolved from thiamin diphosphate (TDP) by acidification of the ammonium sulfate precipitate to pH 3.5, but not by other tested procedures. Resolution was 98% by chemical measurement of residual thiamin and 95% by residual enzyme activity. Reconstitution of the resolved preparation by incubation with TDP was dependent upon TDP concentration, duration, temperature, and the presence of dithiothreitol. At low TDP concentrations, 1 h was required for maximum activation; kinetic analysis then yielded an apparent Km value for TDP of 65 nM (SD 14 nM) from 100 erythrocyte lysates and similar values for reconstituted resolved preparations previously purified 400-fold and 10,000-fold. Velocity data obtained by transketolase assays in which the TDP was added to resolved preparations simultaneously with substrates yielded an apparent Km value for TDP of 2.3 microM (SD 1.6 microM) from 114 erythrocyte lysates and similar values for purified preparations. The recovery of activity following resolution and reconstitution ranged from 21 to 60% from lysates and 38 to 70% from purified preparations. Residual ammonium sulfate up to 4.9 mM decreased the apparent Km value for TDP, while a concentration of 11.3 mM increased the value in a manner competitive with TDP and with an apparent Ki value of 2.3 mM. The spectrophotometric assay of transketolase activity was greatly affected by storage of frozen solutions of the substrate ribose 5-phosphate.

Purification and characterization of, and preparation of an antibody to, transketolase from human red blood cells

Transketolase (sedoheptulose-7-phosphate: D-glyceraldehyde-3-phosphate glycolaldehydetransferase, EC 2.2.1.1) was purified 16 000-fold from human red blood cells, using DEAE-Sephadex A-50, Sephadex G-150, FPLC on Mono P, and Sephadex G-100. The purified enzyme migrated as a single protein band on SDS-polyacrylamide gel electrophoresis. The FPLC step resolved transketolase into three peaks, designated I, II and III. From results of re-FPLC on Mono P, SDS-polyacrylamide gel electrophoresis, gel filtration, catalytic studies, amino acid analysis and immunological studies, it was concluded that I, II and III were originally the same protein, modified during storage and purification. Transketolase had a subunit (Mr 70 000) and appeared to be composed of two identical subunits. 1 mol of subunit contained 0.9 mol of thiamine pyrophosphate. The pH optimum of the reaction lay within the range 7.6-8.0, and the Km values were determined to be 1.5 X 10(-4) M for xylulose 5-phosphate and 4.0 X 10(-4) M for ribose 5-phosphate. Hg2+ and p-chloromercuribenzoate inhibited the enzyme reaction, and the inhibition of the latter disappeared upon the addition of cysteine. Thiamine and its phosphate esters did not, but cysteine (1 X 10(-2) M) and ethanol (10% and 1% v/v) did activate the enzyme reaction. Antibody prepared to II bound all forms of transketolase in the hemolysate, but inhibited the reaction only about 20%.

Ligand-induced interconversions of maize homoserine dehydrogenase among different states

The threonine-sensitive homoserine dehydrogenase has been isolated and extensively purified from shoots of Zea mays L. var. earliking. This enzyme is shown to be hysteretic under certain conditions. Progress curves of the NAD-dependent reaction catalyzed by the maize enzyme can be characterized by distinct lags prior to achievement of steady state velocities, reflecting transitions from less active species to a more active steady state form of the enzyme. Incubation of the enzyme for 1 min at 25 degrees C prior to initiation of the reaction profoundly influences the properties of the less active enzyme and the nature of the subsequent slow transitions during assay. When the feedback modifier, L-threonine, or KCl is included in the preincubation mixture, the transitions involve biomolecular association reactions. In the absence of either ligand, or in the presence of an appropriate mixture of both, a unimolecular transition occurs during assay. Three unique preincubation states of the enzyme have been identified on the basis of their response to substrates and effectors; whereas, the kinetic and regulatory properties of the steady state form of the enzyme are independent of preincubation conditions. Steady state can thus be achieved by three different transitions. Each transition is retarded by threonine and favored by substrates and potassium, although the effects of these compounds differ quantitatively. Under the conditions tested, monovalent cations have no effect on the steady state velocity of the enzyme. A model describing the relationships among the four unique states of the enzyme which is consistent with the present results and supported by previous observations is proposed.

Kinetic studies of mouse brain transketolase

The activity of transketolase in mouse brain was 5.7 nmol/min/mg protein measured by an enzyme-coupled spectrophotometric assay. The apparent Km for ribose-5-phosphate was 330 microM, for D-xylulose-5-phosphate was 120 microM, and for thiamine pyrophosphate was 7 microM. However, thiamine pyrophosphate remained tightly bound to transketolase in homogenates in which it dissociated completely from another thiamine pyrophosphate-dependent enzyme, the pyruvate dehydrogenase complex. These data suggest that loss of transketolase activity is likely to be a later consequence of thiamine deficiency in mammalian brain than is decreased activity of pyruvate dehydrogenase complex.